1. The field of art to which the invention pertains comprises the art of measuring and testing.
2. By and large, the usual type of gas meter registers the actual volume of gas passed therethrough. Even where density changes of the gas are incurred in transmission from varying pressure and/or temperature conditions to which it is exposed, these changes are largely ignored. Yet gas being a compressible medium, the quantity by weight passing through a meter is a function not only of its volume but also its pressure and temperature in accordance with Boyle's and Charles' laws. Failure to correct for these deviations can therefore introduce substantial error in the readout value of the commodity being sold. In the instance of a combustible gas being supplied as a fuel, it should be recognized that the saleable commodity thereof is in its calorific content per unit of weight rather than volume.
Where cost charges are predicated on volume alone, a density increase will ordinarily cause the supplier to incur an economic loss whereas a density decrease will ordinarily cause the recipient to incur an economic loss. As can be appreciated, neither situation is equitable in terms of the real commodity being marketed. In recognition of the foregoing and with the objective of obtaining a more correct and equitable charge basis to both the supplier and recipient of the metered gas, various efforts have been made over the years to correct from the actual volume to an equivalent standard volume. Combined pressure and temperature correction apparatus has been in the form of independent instruments adapted to mount standing alone externally to the gas meter which provides a standard instrument drive interface. Exemplifying prior art gas meter compensating apparatus for pressure are the disclosures of U.S. Pat. Nos. 2,093,151 and 2,122,529 whereas exemplifying apparatus for correcting temperature is disclosed by U.S. Pat. Nos. 2,791,118 and 3,581,566. A density measuring device integrating both temperature and pressure variables is disclosed in U.S. Pat. No. 2,601,777.